1. Technical Field
The present invention relates to battery chargers in general, and in particular to a method for suppressing heat generated by a semiconductor switch within a battery charger.
2. Description of Related Art
Battery chargers employed for charging a battery with a DC/DC converter generate a DC output voltage from a DC input voltage by controlling a semiconductor switching device such as a MOS-FET in a Pulse Width Modulation (PWM) manner or a Pulse Frequency Modulation (PFM) manner. Under PWM, the ON duration of a switching device is controlled so that a switching frequency as a function of a period corresponding to the total duration of the ON duration and the OFF duration is made constant. Under PFM, the OFF duration is controlled while making the ON duration constant in order to change a switching frequency, thus controlling an output voltage within a certain range.
In addition to a switching device, a battery charger also includes an integrated circuit device for control, an inductor and a smoothing capacitor. The switching device is required to suppress a temperature rise due to heat generated during operation within a predetermined range. The amount of heat generation by a switching device depends on constants unique to the switching device such as ON resistance, gate charge capacitance, turn-on/turn-off time, circuit constants such as an input voltage and an output current (charge current) and switching frequency.
Battery chargers are typically designed by choosing a switching device for the input voltage, and the output voltage dictates the switching frequency. Since a DC/DC converter includes a ripple component in the output voltage in theory, permissible ripple content also needs to be considered for the battery charger design. The ripple content increases with a decrease in the switching frequency. Thereafter, an inductor suitable for output current is selected, and finally a capacitor for placing a ripple voltage within a permissible range is selected. The amount of heat generation decreases with a switching frequency, and therefore a compact switching device can be used. However, ripple current will also increase, which results in an increase in the sizes of the inductor and the capacitor in order to control the ripple voltage within a predetermined range.
Lithium batteries are charged in a constant voltage/constant current manner. Lithium batteries have to be charged while strictly controlling the upper limit of a charge voltage to a battery cell so as to keep the safety and suppress the degradation, and so an overvoltage protection circuit is provided to control the maximum value of the charge voltage. While battery charging is being performed in a constant voltage area, the output voltage of a battery charger reaches a maximum value. Thus, when a large ripple voltage superimposed on the output voltage, the overvoltage protection circuit will begin to operate.
The output voltage in the constant voltage area can be lowered in order to keep the overvoltage protection circuit from start operating even when the ripple voltage increases. However, it is not preferable to have the output voltage being lowered more than what's required because the battery will not be able to reach a full charge capacity at the completion of the battery charging process. Therefore, conventional battery chargers using a DC/DC converter in a PWM manner tend to maintain a switching frequency at a certain high value so as not to let an overvoltage protection circuit operate from the battery charging period at a constant voltage area to the charging period at a constant current area.
Recently, lithium batteries capable of being charged in a relatively short time using a large charge current have become available. Since the battery chargers for quickly charging such quick-charge type lithium batteries have to supply a large current, heat generated due to power loss of a switching device in a DC/DC converter begins to present a problem. In order to cope with this problem, a large-capacity switching device may be used, which introduces another problem of an increase in size and cost of battery chargers.